The present invention relates to a liquid crystal display device. More specifically, the present invention relates to a liquid crystal display device having a wide viewing angle characteristic.
A liquid crystal display device is a flat display device with advantageous features such as a reduced thickness, a reduced weight, a reduced power consumption, etc. However, a problem of a liquid crystal display device is that the appearance of the displayed image varies depending upon the direction from which it is viewed, i.e., the “viewing angle dependency” is substantial. The substantial viewing angle dependency of a liquid crystal display device is primarily due to the fact that liquid crystal molecules having a uniaxial optical anisotropy are uniformly oriented in the display plane.
An effective method to improve the viewing angle characteristic of a liquid crystal display device is to produce a so-called “multi-domain orientation” by forming a plurality of regions of different orientations within a picture element region. Various methods have been proposed in the art to realize a multi-domain orientation. Typical methods, among others, for realizing a multi-domain orientation in a liquid crystal display device of a vertical alignment mode include those disclosed in Japanese Laid-Open Patent Publication No. 6-301036 and Japanese Laid-Open Patent Publication No. 11-258606.
Japanese Laid-Open Patent Publication No. 6-301036 discloses a method in which an opening is provided in a counter electrode which opposes a picture element electrode via a liquid crystal layer interposed therebetween so as to control the orientation direction of the liquid crystal molecules by utilizing the inclination (bending) of an electric field in the vicinity of the region where the opening is provided. An inclination of an electric field means the production of an electric field component parallel to the substrate plane (the plane of the liquid crystal layer). Therefore, the direction in which liquid crystal molecules having a negative dielectric anisotropy (which are in a vertical alignment in the absence of an applied voltage) are inclined in the presence of an applied voltage (i.e., the azimuth angle direction: the direction of the long axis of inclined liquid crystal molecules as it is projected onto the substrate surface) is defined by the electric field component parallel to the substrate plane. In other words, a component of an inclined electric field which is parallel to the substrate plane exerts an orientation-regulating force.
Japanese Laid-Open Patent Publication No. 11-258606 discloses that a multi-domain orientation can be obtained by forming a protrusion, a depression or a slit (an opening provided in an electrode) on one surface of each of a pair of substrates opposing each other via a liquid crystal layer interposed therebetween (e.g., a TFT substrate and a color filter substrate) which is closer to the liquid crystal layer. With either one of the methods disclosed in these publications, it is possible to realize a desirable viewing angle characteristic by using the method in combination with an appropriate optical compensation film.
However, a study conducted by the inventor of the present invention revealed that the techniques disclosed in the publications above have the following problems.
With the method of Japanese Laid-Open Patent Publication No. 6-301036, it is difficult to obtain a uniform multi-domain orientation. Moreover, when the value of the voltage applied across the liquid crystal layer is changed, it takes a relatively long time to complete the change in the orientation according to the change in the voltage value; that is, the response speed is slow. It is believed that these problems are caused by the fact that the orientation-regulating force for achieving a multi-domain orientation (a force for orienting the liquid crystal molecules in a particular direction) is relatively weak in this method.
The problems as described above do not occur with the method of Japanese Laid-Open Patent Publication No. 11-258606. It is believed that with this method, a sufficiently strong orientation-regulating force is obtained, thereby realizing a relatively stable multi-domain orientation. However, this method has the following problems.
First, if the method disclosed in this publication is employed, it is necessary to provide a protrusion, a depression or a slit on both of the pair of substrates interposing the liquid crystal layer therebetween in order to achieve a multi-domain orientation, thereby complicating the production process of the liquid crystal display device and lowering the production efficiency.
In a plasma-addressed liquid crystal display device (hereinafter, referred to as a “PALC”), a thin glass plate (which forms a part of a plasma cell substrate) having a thickness on the order of 10 μm and an area on the order of 1 m2 is arranged on the side of the liquid crystal layer. Therefore, it is difficult to provide a protrusion or a depression on the surface of the plasma cell substrate which is closer to the liquid crystal layer. Moreover, although the thin glass plate itself functions as an electrode (it is sometimes called a “virtual electrode”), it is not an electrode made of a conductive layer, whereby a slit (an opening provided in an electrode) cannot be provided therein. Therefore, it is very difficult to use the method disclosed in this publication with PALCs.
With the method of this publication, the orientation-regulating force can be increased by miniaturizing an orientation region by narrowing the interval between adjacent protrusions, depressions or slits, which defines the orientation region. However, since the positional precision of the orientation regions depends directly upon the precision of the attachment of the substrates with each other, the orientation region cannot be miniaturized excessively.